In peak load systems such as printers and point-of-sale (POS) systems, an output power Po of a power converter rises suddenly to form a surge in time of operation as shown by the waveform in FIG. 1. Thus, a proper protecting mechanism is needed to determine whether the surge is normal or is resulted from the abnormal output power. A conventional determining method for the abnormal output power is to observe whether a duration time of the surge is over a preset value (about 100 ms-1 s).
U.S. Pat. No. 7,486,493 discloses a method for determining whether the output power of a power converter is abnormal or not. FIG. 2 shows the operation of this art, in which two thresholds VT1 and VT2 are preferably set, a sensing signal VCS related to the current flowing through a power switch at the primary side of a transformer is utilized to determine the output power Po, and the first threshold VT1 is used for limiting the maximum of the output power Po. When the sensing signal VCS equals to or higher than the first threshold VT1 as shown at time t3, it means that an output short occurs, and the power converter will be turned off right away. When the sensing signal VCS is higher than the second threshold VT2 and lower than the first threshold VT1 as shown at time t1 to time t2, a timer starts counting. If the duration time Ts of the sensing signal VCS that is higher than the second threshold VT2 is lower than a preset time Tth, it means that the output power Po of the power converter is normal. Oppositely, if the duration time Ts equals to or higher than the preset time Tth, it means that the output power Po of the power converter is abnormal, and the power converter is to be turned off.
However, the aforementioned art utilizes a same clock to generate a driving signal for controlling the power switch and count the duration time Ts that the sensing signal VCS is higher than the first threshold VT1. When the power converter enters a light-load mode, it needs to decrease the frequency of the clock for lowering the switching times of the power switch in order to lessen the switching loss. However, afore operation cannot count the duration time Ts correctly because the timer usually employs pulsed quantity of the clock to determine the time. If the period of the clock is 1 ms and the preset time Tth is 100 ms under a heavy-load mode, the timer is set for determining an abnormal output power while the 100th pulse is counted. However, in time of the light-load mode, the period of the clock becomes 2 ms. Accordingly, the timer counts the fiftieth pulse under the light-load mode, but the actual duration time Ts is over the preset time Tth. Nonetheless, the timer does not determine an abnormal output power until it counts the 100th pulse. As a result, the power converter cannot be turned off in time, which results in a damaged power converter. Moreover, the sensing signal VCS responds to the output power in real time. Namely, in time of counting the duration time Ts, the timer counts the time Ts again if the output terminal of the power converter is interfered, the output power Po varies severely such that the sensing signal VCS drops below the second threshold VT2 in seconds as shown at time t4. Thereby, the power converter is not turned off at time t5 but delayed until time t6, which easily causes the damaged power converter.
Today, there are methods that the sensing signal VCS will not be easily interfered by the transient of the output power. For example, the datasheet UCC 28630 discloses that an average of the sensing signal VCS is acquired, so that the interference of the transient of the output will not affect the average thereof easily. But it should be noted that all the present methods require a complicated circuitry.